Dr. Sanjay Gupta tests the Apple Watch's new ECG heart monitor

Feature available with software update

(CNN) - Since the Apple Watch Series 4 was announced in September, one of its most anticipated functions has been the promise to perform a mobile electrocardiogram (ECG or EKG) whenever a consumer wants. The feature becomes available in the United States on Thursday via a software update.

This new ECG app represents a big venture into health for Apple. The company's ambition is for Apple and health to become as closely linked as Apple and music. It even produced a commercial featuring people who say the Apple Watch has helped them lead healthier lives.

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Apple says its new ECG application and the ability to notify the user of an irregular heart rhythm have been in the works for years. It was an outpouring of feedback from users of the Apple Watch heart rate trackers that prompted the initial serious discussions into directly measuring the heart's electrical system.

According to Apple, "thousands of emails and letters" arrived -- many addressed to CEO Tim Cook -- telling stories of individuals finding an abnormal heart rate on their Apple Watch, following up with a doctor and getting necessary -- sometimes life-saving -- care. It was this feedback that left an impression on Apple executives at the highest levels of the company and inspired the app.

But Apple is not the first to offer an over-the-counter consumer ECG device. A company called AliveCor released a version of this technology in November 2017, called KardiaBand, that can be used with previous generations of the Apple Watch.

Apple's new ECG app is not dependent on a separate band. It uses new electrodes built into the sapphire crystal on the back of the watch as well as the digital crown, the wheel on the side of the watch. Unlike AliveCor's products, the Apple ECG does not require a subscription or a doctor's involvement.

AliveCor says it plans to release an updated version of its technology in April, which will be a six-lead ECG reader, as opposed to Apple's current release, which is a one-lead ECG.

Not all ECGs are the same

When you hear the term ECG or EKG, it is usually in reference to a 12-lead echocardiogram, the gold standard for measuring the heart's electrical pattern. A 12-lead ECG is the professional version that is administered in a clinic or hospital setting. Twelve leads (using 10 electrodes) are placed on your limbs and across your torso. They measure the heart's electrical activity in several directions and planes and are highly sensitive. They can detect abnormal electrical patterns and find areas where heart muscle is dying, as in the case of inadequate blood supply or heart attack.

Apple's new app is a much simpler single-lead ECG with limited capabilities. It won't help detect most heart rhythm abnormalities or worsening heart failure. It also won't reliably detect the electrical changes associated with a heart attack. Apple concedes this and provides plenty of labeling on the app and in the accompanying literature cautioning against overinterpreting the results.

It should go without saying: If you think you are having a heart attack, you should always call 911 first before fussing with your Apple Watch or any monitoring device, no matter how good the technology.

The most notable capability of the new ECG app is helping detect a condition known as atrial fibrillation, an irregular heartbeat that could be described as quivering. It can be a serious condition, leading to blood clot formation, strokes and heart failure. Patients may feel a "fluttering" in their chest, but symptoms can dissipate before they can get to a doctor for an ECG, making it challenging to conclusively diagnose.

The US Centers for Disease Control and Prevention estimates that as many as 6.1 million Americans are living with a-fib, and that number is only expected to increase as the population grows older. Apple's hope is that with the single-lead ECG available anytime you are wearing your watch, you have a better chance at screening for atrial fibrillation at the time it is happening.

I found the process of using the ECG app remarkably easy, but there are a couple of important caveats. You must be dry, sitting or standing still, and able to hold your hands in place for 30 seconds. That is to say, an electrocardiogram cannot be obtained while you are running or doing another strenuous activity, which is known as a "stress EKG."

Once you open the app on your watch, you touch the digital crown with your free hand to close the "electrical loop" in your body, sending a small amount of current through your finger. You will immediately see your heart rhythm strip appear on the watch screen, and after 30 seconds, your iPhone alerts you that your ECG is available to review. Since most people are unable to interpret an ECG reading, a description of the findings also pops up on the screen, primarily indicating whether you are in a sinus rhythm, which is normal; whether you are in abnormal atrial fibrillation; whether your heart rate is abnormally low or high; or if the test is inconclusive.

As is the case with most screening tools, the concern is incorrectly identifying conditions that aren't present and/or missing conditions that are, known as false positives and false negatives. One is a nuisance that can lead to anxiety, additional testing and medications. The other can be deadly.

Apple assures me it has given this issue significant consideration and has reached out to the medical community throughout the design process to help address it. In one study, Apple tested the app in nearly 600 people, half of whom had atrial fibrillation and half who did not. When this study group used the watch, the result was inconclusive about 12% of the time. The rest of the time, however, the app was able to accurately tell people they did not have a-fib 99.6% of the time and accurately tell them they did have a-fib 98% of the time, according to the study.

This is not for everyone

The Apple Watch is cleared, but not approved, by the US Food and Drug Administration. It may seem a subtle point, but for something to be approved by the FDA, it is subjected to much more rigorous testing and data collection. Clearance is typically given to medical devices that have been determined to be substantially equivalent to another legally marketed device -- an easier hurdle to cross.

As people start using the app over the next several days, there will probably be certain glitches that become apparent, as with most tech releases. The app also raises questions about privacy, though Apple maintains that ECG data is stored only on your phone and that you are the only one who can choose to share it.

The app may also increase visits to the doctor from newly concerned patients. Still, there has been considerable enthusiasm from the medical community as a whole.

When Apple announced the ECG app at a special product event in September, Dr. Ivor Benjamin, president of the American Heart Association, took the stage and said "capturing meaningful data about a person's heart, in real time, is changing the way we practice medicine."

There is no doubt Apple is counting on doctors to use the data collected by the Apple Watch. The company has made it easy to upload your ECG, along with a description of your symptoms, to your personal doctor directly from the app to facilitate that communication. It's all part of their big bet on making an impact in health care.

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